Metallocene catalysts for olefin polymerization have been developed over a long period of time. Generally, metallocene compounds have been used after activated with aluminoxane, borane, borate, or other activators. For example, in the case of a metallocene compound having a ligand including a cyclopentadienyl group and two sigma chloride ligands, aluminoxane is adopted as an activator. It was reported that replacement of the chloride group with another ligand (e.g. benzyl or trimethylsilylmethyl (—CH2SiMe3)) in the metallocene compound exhibits effects of increased catalytic activity or the like.
EP 1462464 discloses an example of polymerization using a hafnium metallocene compound having chloride, benzyl and trimethylsilylmethyl groups. Also, results were reported that energy generation of active species varied depending on the type of alkyl ligand coupled with a central metal (J. Am. Chem. Soc. 2000, 122, 10358). Korean Patent No. 820542 discloses a catalyst for olefin polymerization having a quinoline-based ligand, and this patent pertains to a catalyst having a living group including silicon and germanium atoms in addition to the methyl group.
In the early 1990s, [Me2Si(Me4C5)NtBu]TiCl2 (Constrained-Geometry Catalyst, CGC) was disclosed by DOW in U.S. Pat. No. 5,064,802. In copolymerization of ethylene and alpha-olefin, CGC is superior to metallocene catalysts known to date in terms of (1) high activity even at high polymerization temperature to produce a polymer having a high molecular weight, and (2) very high ability to copolymerize alpha-olefin having high steric hindrance, such as 1-hexene and 1-octene. In addition, a variety of characteristics of CGC are gradually known upon polymerization, and thus ongoing research on synthesis of derivatives thereof to use as a polymerization catalyst has been conducted in academic and industrial fields.
As one approach, it was attempted to synthesize metal compounds including a nitrogen substituent and a variety of other bridges instead of a silicon bridge and to perform polymerization using the same. Representative metal compounds were known to contain phosphorus, ethylene or propylene, methylidene and methylene bridges, in lieu of the silicon bridge of the CGC structure, but did not exhibit superior polymerization activity or copolymerization performance when applied to ethylene polymerization or copolymerization of ethylene and alpha-olefin, compared to CGC.
As another approach, a large number of compounds having an oxido ligand instead of the amido ligand of CGC were synthesized, and there were some attempts to perform polymerization using the same.
However, when a propylene-based elastomer having a narrow molecular weight distribution is prepared by using the known Group 4 transition metal catalyst, it is difficult to prepare the propylene-based elastomer having a high density in the same comonomer content and excellent mechanical properties with high productivity and yield.